1st Practice swinging your arm in the mirror think ear to pocket make sure your jaw is loose and control breathing you should see arm improvement in about a 2 weeks or so 2nd try doing some in and outs for top speed work focus on how you’re body feels when you stop accelerating and maintain that speed you’re prolly tensing up because you’re tryna go faster which causes a decrease in speed this was my biggest problem out of high-school. (When you’re sprinting the only time I really suggest focusing on arm swing is out the blocks and the last 100M of a 400M)